The present invention relates to blind fasteners for securing workpieces of a crushable or composite material and more particularly to a blind fastener providing a blind head with a large effective bearing surface on the blind side of such workpieces.
In certain applications, especially those involving workpieces of crushable or composite materials, it is desirable to utilize a blind fastener providing a large bearing area under the blind head. The large bearing or surface area around the workpiece opening on the blind side assists in inhibiting localized crushing and flaking of the workpiece material engaged by the blind head.
Fasteners generally of the type of the present invention are shown in U.S. Pat. No. 3,345,900 to J. P. Villo issued Oct. 10, 1966, U.S. Pat. No. 4,033,222 to L. B. Wilson, issued July 5, 1977, U.S. Pat. No. 4,168,650 to W. F. Dahl, et al, issued Sept. 25, 1979, U.S. Pat. No. 4,457,652 to J. D. Pratt, issued July 3, 1984 and U.S. Pat. No. 4,203,346 to D. E. Hall et al, issued May 20, 1980.
The preceding patents generally disclose a multipiece fastener including a main sleeve or nut member and an expandable sleeve which forms a blind head as it is axially moved by a pin member over the blind end of the main sleeve. The '900 patent to Villo generally shows this type of structure but the resultant fastener does not form a blind head having a large bearing area or "footprint". The '222 patent to Wilson shows a structure including three expandable sleeves utilized in an effort to provide a blind head having a large bearing area. The '650 patent to Dahl et al and '652 patent to Pratt each show an expandable sleeve having a reduced section portion at the leading end which bulbs radially outwardly to ostensibly provide a large bearing area. A variation of these structures is shown in the '346 patent to Hall et al where the reduced section portion engages a shoulder on the pin member.
Frequently fasteners of this type have expandable sleeves which are solution treated, annealed or otherwise softened to a reduced, uniform hardness to minimize installation forces on the blind side of the workpiece of crushable or composite material. In some cases, the expandable sleeve has a primary section of one wall thickness or cross-section and a secondary section of reduced wall thickness or cross-section. The main sleeve or nut member has a tapered nose portion at its blind side. In setting the fastener, the expandable sleeve is radially expanded as it moves axially over the tapered nose portion via a relative axial pulling force applied between the pin member and the nut member. The blind head is formed by folding of the secondary section as it engages the workpiece surface. One of the problems with such fasteners is that the softened, expandable sleeves do not consistently provide, if at all, a blind head having the desired large bearing area against the engaged workpiece surface. This is due to radial inward movement of the leading end of the primary sleeve section as it moves axially over the tapered nose portion of the main sleeve. It is believed that this deformation occurs in response to hoop stresses resulting from the initial radial expansion.
In the present invention, it has been found that a large blind side bearing area can be achieved by providing an expandable sleeve structure having its primary section to be of a higher hardness than the secondary section and thus to have a greater resistance to hoop stresses imposed during installation; as noted it is the hoop stresses present during installation which cause radial inward deformation of the softened sleeves of the prior art. Thus the magnitude of the lower hardness of the secondary section will be selected to promote folding for formation of the blind head at the desired installation load; at the same time, the magnitude of the higher hardness of the primary section will be selected to inhibit radial inward deformation of that section resulting in the formation of the blind head with large bearing area by the secondary section.
The fastener of the present invention is constructed to be set by a relative axial force applied between the pin on the one hand and the main sleeve and expandable sleeve on the other hand whereby the expandable sleeve is moved over the tapered blind end or nose portion of the main sleeve. In this way the secondary sleeve section will be radially expanded first and moved axially until its free end engages the blind side of the workpieces. The secondary sleeve section is and remains integrally connected to the primary sleeve section; when the axial or column load attains a preselected magnitude the secondary sleeve section will fold to form a blind, bulbed head having a large effective bearing area whereby localized crushing of the engaged workpiece is inhibited. The relative axial load continues to increase until the pin fractures at a breakneck groove severing a pintail portion of the pin leaving a pin shank of a predetermined minimal length.
During the installation, the leading end of the primary sleeve section will be radially expanded as it moves axially over the tapered nose portion of the main sleeve. The higher hardness, and hence strength, of the primary section will resist and substantially preclude radial inward deformation of that section throughout the installation whereby the final formation of a blind head of desired large bearing area by the softer secondary sleeve section will be promoted. By contrast, with prior, solution treated or softened sleeves the blind head in some conditions can be partially deflected away from the engaged workpiece surface by the radially inward movement of the primary sleeve section thereby reducing the effective size and integrity of the footprint of the blind head.
Fasteners of the type of the present invention are provided in different diameters and in different lengths to accommodate workpieces varying in thickness. The nominal total thickness of workpieces to be secured by a fastener of a given length is considered to be the "grip" of that fastener. It is desirable that a fastener of such given length or "grip" can be used to clamp or secure workpieces varying in total thickness over a predetermined range. The noted range is considered to be the "grip range" of the fastener. With the fastener construction of the present invention, the desired large `footprint` and hence large effective engaged area of the blind head can be provided by the fastener of a given length (or grip) for joining workpieces varying in total thickness over a considerable grip range. In this regard, it is also desirable that the installed fastener provide an indication that the fastener with proper grip has been installed with workpieces within its grip range; the most effective means of indication of the use of a fastener of correct or incorrect grip is the final position of the pin. With prior constructions of generally uniform low hardness, selected to reduce the load to fold and collapse the secondary sleeve section, it was possible for the primary softened sleeve section to buckle. When such buckling of the primary sleeve section occurs, it is not possible to use the final position of the pin on the open or installation side to indicate or flag the correctness of the grip of the fastener. Such buckling of the primary sleeve section is substantially eliminated with the present invention resulting in a fastener having the capability of indicating or flagging the correctness of the grip in which it is used.
In this regard, it should be noted that with the solution treated or softened sleeves the buckling could be such that the axial travel of the pin would continue until the pin head engaged the nose or tapered end of the main sleeve or nut member. When this occurs the operator or inspector on the open side of the workpieces, by visual inspection, could erroneously conclude from the overtravel of the pin that a fastener with incorrect grip was used. In other circumstances the operator or inspector could fail to detect that the fastener was used in an incorrect grip. With the present invention the total amount of pin travel is controlled since the primary section of the expandable sleeve will not buckle; thus, over the selected grip range of the fastener of a given grip, the pin break location will occur near the outer sheet line or surface of the outer workpiece with a maximum grip condition and at a determinable distance beyond the outer sheet line with a minimum grip condition. Any deviation from these extremes will indicate or flag an improper or undesirable grip condition for the set fastener. In this way, the present invention provides a visual and/or measurable indication that the fastener of proper grip has been used.
Another problem with overtravel of the pin with solution treated or fully softened constructions is the potential for breaking the pin head off as it engages or stalls near the nose of the main sleeve; this is avoided by the controlled pin travel provided by the present invention.
Fasteners of this type are utilized to secure workpieces being predominantly subjected to shear loads rather than tensile loads. In a lap shear type joint, however, there is also a twisting or bending load being applied which has a tensile component. Thus the magnitude of shear load capacity of such fasteners will be influenced by its ability to withstand the combination loading applied resulting from twisting and/or bending loads. In the present invention, the resultant larger footprint of the blind head plus the higher hardness of the primary sleeve portion and its general freedom from buckling provides a fastener construction having a greater resistance to the twisting or bending loads and hence provides a higher shear load capability than fasteners utilizing the softened construction.
Thus it is an object of the present invention to provide a blind fastener for fastening workpieces of a crushable or composite material and which provides a blind head having a large bearing area against the adjacent blind side of the workpieces.
It is another object of the present invention to provide a blind fastener of the above described type having a main sleeve and an expandable sleeve with the expandable sleeve having a secondary section of reduced cross-section which is of a lower hardness than the remainder or primary section of the expandable sleeve such that the secondary sleeve section can fold to form the blind head while the primary section can resist radial inward deformation at the axial loads necessary to set the fastener whereby the blind head will be formed with a desired large bearing area.
It is still another object of the present invention to provide a blind fastener of the above described type in which buckling of the primary sleeve section is precluded whereby the travel of the pin over the selected grip range of the fastener is predictable for a proper installation thereby providing a visual and/or measurable indication of the correctness of the grip in which the fastener is set.
It is another general object to provide a new and unique blind fastener constructed to provide a blind, bulbed head having an enlarged bearing area.
Other objects, features, and advantages of the present invention will become apparent from the subsequent description and the appended claims, taken in conjunction with the accompanying drawings.